The invention relates to a milling blank for the production of medical-technical molded parts, in particular dental splints or ear molds, as well as a method for the production of such a blank.
Blanks for the production of medical-technical molded parts of the initially named type are known in various designs from the prior art. Both dental splints as well as ear molds are currently produced substantially by means of two different methods of the prior art.
In the case of the first method known from the prior art, an impression of the positive (tooth crown or ear canal) is taken in a first step. Then, in the case of a dental splint, a positive plaster model is produced from the impression, on which then e.g. a splint (negative) is deep-drawn or a 2-component material is applied by means e.g. of a spreading technology and is then hardened. In the case of the so-called PNP process (positive-negative-positive) for the production of ear molds, the hearing aid acoustician takes in a first step an ear impression (positive) for the production of an otoplasty (for devices worn behind the ear) or a shell (for devices worn in the ear). In a second step, a negative mold (N) is prepared by means of the impression, into which either a radiation-curable or an autopolymerizing, low-viscosity formulation is subsequently poured. It is then hardened by means of heat in a pressure pot or by means of radiation.
The dental splint (negative) prepared in this manner or the ear mold (positive) must be optimally fitted for the anatomical conditions. Inaccurate molds would otherwise cause discomfort (e.g. pressure points, poor hold) and impair the function of the splint or hearing aid (e.g. misaligned teeth/feedback). As a result, it is important that the formulation has the lowest viscosity possible so that even undercuts and the finest surface textures are filled in with material and can be formed as true to detail as possible.
Additive layer processes such as e.g. stereolithography, are used as another method group of the prior art for the production of splints/ear molds, which functions based on digital data. It is thereby known from publication U.S. Pat. No. 4,575,330 that low-viscosity, radiation-curable resins or respectively resin mixes can be used for the production of three-dimensional objects by means of stereolithography. Furthermore, it is known from publications U.S. Pat. No. 5,487,012 and WO 01/87001 that the stereolithography can be used advantageously for the production of ear-pieces.
In the case of the stereolithographic method, three-dimensional objects made of a low-viscosity, radiation-curable formulation are structured in a manner that respectively one thin layer (approx. 25-100 μm) of the formulation is precured by means of actinic radiation in a defined manner such that the created layer has the desired cross-sectional shape of the object at this position. The created layer is simultaneously polymerized on the layer cured in the previous step. The structure of the overall object can thus be accomplished with the help of a computer-controlled laser system such as e.g. an Nd:YVO4 solid-state laser (Viper si2 SLA System, 3D Systems, USA). The generated mold is postcured, if necessary, e.g. through radiation.
Special demands are made of the resin formulations that can be used in the stereolithographic process. In particular, the radiation sensitivity and the viscosity of the resin formulations as well as the strength of the molds precured by means of the laser curing should thereby be named. This not fully cured mold is called a green compact in stereolithographic technology and the strength of this green compact, characterized by the E module and the bending strength, is called green strength. The green strength is an important parameter in the practical application of stereolithography, since molds with a low green strength are deformed under their own weight during the stereolithography process or can sink or bend during the postcuring, for example with a xenon arc or halogen lamp.
Furthermore, for process-related reasons, the green compacts are built on supporting structures called supports. These supports must position the green compact in a stable manner during the entire production process since the position of the green compacts must not change due to the coating process. Accordingly, the supports for a stereolithographic process can only have a minimal flexibility.
For all of these reasons, it is only possible in a very limited manner to generate flexible ear molds on the basis of three-dimensional data. For one, it is necessary for the stereolithographic process to use the lowest-viscosity resins (<3 Pa s) possible. For this reason, certain material classes, such as silicone materials or highly filled composites, are not accessible or only accessible to a very limited degree.
This also applies to systems that have a so-called temperature-induced memory effect. However, this effect is useful for many medical-technical applications and is even essential for new applications. For example, a dental splint can be twisted during insertion in the mouth. Through the memory effect induced by the body heat, it is then molded back into the optimal position while it is worn. This considerably increases the wear comfort and prevents the generation of defective positions in comparison to a hard, deformed material.